The present invention relates to a fixing apparatus and an image forming apparatus using the fixing apparatus to fix a toner image (picture) onto a fixing material in image forming apparatuses such as an electrostatic, a laser printer, and the like.
A fixing apparatus installed in the copying machine using electrophotographic processes heats and melts a developer, i.e., toner, formed on a fixing material and fixes the toner on the fixing material.
Many of this type of fixing apparatuses form a working section (nip) at a position where a heating roller or an endless belt contacts with a press roller supplying the heating roller (belt) with a specified pressure. There is widely used a method and a configuration for supplying pressure and heat to the fixing material and toner guided by the nip. Recently, as a toner heating method available for the fixing apparatus, the method using the induction heating is becoming widely used for the sake of a short wait time and low power consumption.
For example, Jpn. Pat. Appln. KOKAI Publication No. 10-63126 discloses the fixing apparatus to heat the roller using the induction heating.
As another example, Jpn. Pat. Appln. KOKAI Publication No. 8-76620 discloses the fixing apparatus that uses a magnetic field generating means to heat a heat-resistant conductive film and fixes toner on paper adhered to the conductive film. The same publication discloses an example of providing a nip by putting the conductive film between the magnetic field generating means and the heating roller.
The fixing apparatus using the heating roller widely uses a technique of thinning a metal roller in order to shorten the time needed for warm-up. In addition, a technique of thinning rubber layers is used for the roller whose surface is provided with a rubber layer so as to be appropriately used for a color image forming apparatus, for example.
Since thinning the rubber layer of the heating roller increases the heating roller hardness, it is proposed to increase the nip width for the purpose of improving a fixing property. For this reason, there is a tendency to soften the surface rubber layer of the press roller that contacts with the heating roller.
Decreasing the press roller hardness curls a fixing medium toward the heating roller. Accordingly, there is reported an example of allowing a releasing mechanism using a claw to contact with the heating roller surface in order to completely separating a fixing medium from the heating roller.
When the releasing mechanism uses the claw, a scratch due to the claw occurs on a toner image, i.e., an image on the fixing medium. Further, there is a problem of causing a release failure, a jam (entanglement), and the like due to toner adhered to the claw. The releasing mechanism using the claw offers a new problem of removing the toner adhered to the claw at a specified interval (for the specified number of image forming operations).
In order to decrease running costs for the fixing apparatus, there may be a case of omitting a cleaner to remove toner remaining on the heating roller surface or applying no release agent (oil) to the heating roller surface. It is impossible to reliably separate a fixing medium used for a color image that especially causes a large toner coating mass.
In addition to the above-mentioned problems, the following occur when using the induction heating method that heats the roller itself by means of an eddy current generated on the metal due to the electromagnetic induction. The induction heating causes magnetic field distribution dependent on coil shapes. Due to an effect of the magnetic field distribution, the surface temperature of the heating roller partially differs. Further, the fixing apparatus using the induction heating easily causes areas generating partially different temperatures along a longer direction of the heating roller depending on the coil arrangement.